The use of spinal fixation instrumentation to align and/or fix a desired relationship between adjacent vertebral bodies is well established. Such instrumentation typically includes a spinal fixation element, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the element to screws which have been inserted into the patient's vertebrae or to spinal hooks which can be placed into a vertebral arch for coupling to the vertebral bodies. Once installed, the spinal fixation instrumentation holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
One example of a rod based spinal fixation system is provided in U.S. Pat. No. 5,005,562, issued Apr. 9, 1991 to Cotrel (which is hereby incorporated by reference). This system includes pedicle screws and spinal hook vertebral coupling elements (both screws and hooks) having integral U-shaped bodies that extend outward from the vertebrae to which they are attached. A spinal fixation rod is shaped as desired and fitted into the “U” of U-shaped bodies of adjacent vertebrae. The inner surfaces of the U-shaped body are threaded to accept a set screw, and rod is fixed to the vertebral coupling elements by threading a set screw into each of the U-shaped bodies to lock in the rod.
U.S. Pat. No. 5,545,165, issued Aug. 13, 1996 to Biedermann et al. (and incorporated herein by reference), illustrates an improvement in closure systems for fixing a rod to vertebral coupling elements over those provided by Cotrel. The Biedermann et al. system also uses pedicle screws and spinal hooks having U-shaped bodies that extend outward from the vertebrae to which they are attached. The U-shaped bodies of the Biedermann et al. system are threaded on both the inside and the outside. The rod is therefore locked in by both an inner set screw and an outer lock nut. In the illustrated embodiments, the inner set screw is adapted to be driven on its threads using a hex-shaped driver element, and the outer locking nut is provided with hex-shaped flat outer surfaces suitable for engagement with a wrench or similar driving tool.
U.S. Pat. No. 5,443,467, issued Aug. 22, 1995 to Biedermann et al. (and incorporated herein by reference) illustrates the use of an inner set screw and an outer lock nut to lock a rod into a U-shaped body in a polyaxial screw system. In this system, a pedicle screw having a spherical head is captured within a separate U-shaped receiver body. The angle of the screw with respect to the body can be changed until a head-locking element is tightened to lock the angle of the screw head within the receiver body. According to Biedermann et al., this combination of an inner set screw and an outer locking nut provides an advantage in that the force acting on the rod can be independently adjusted by either the inner set screw or the outer locking nut—a particularly useful advantage where the rod being fastened is curved and an exact fastening might only be possible by independent adjustment of the two closure elements. In addition, when tightened, the inner set screw and the outer locking nut tend to lock each other in their tightened positions.
The effectiveness of these spinal fixation systems and others depend upon the rod receiving screws being properly placed. If the rod receiving screws are out of line, the surgeon will be able to place the rod within the receiving portions of the screw assemblies only by placing a load on the screw in order to move it into line to accept the rod. Such loads can result in improper alignment of the vertebrae, premature screw pull out, and fracture of the bone to which the screw is attached.
Accordingly, there is a need for a system, method and/or device to allow the surgeon to place the rod receiving screws in proper alignment before the rod is fitted to them.